CN1341221A - Improvements in or relating to photonic crystal fibres - Google Patents

Abstract

The photonic crystal fiber comprises a bulk material having a longitudinal bore (130,140) and a guiding core (135), wherein the fiber has at least two-fold rotational symmetry about a longitudinal axis, and the fiber is birefringent as a result of the lack of symmetry.

Description

Improved or related to photonic crystal fiber

The invention relates to a photonic crystal fiber and a production method of the photonic crystal fiber.

Photonic crystal fiber is a special form of optical fiber. Optical fibers are used in many fields, including communications, laser processing and welding, laser beam and power delivery, fiber lasers, sensors and medical diagnostics and surgery. Optical fibers are typically made entirely of a solid transparent material such as glass, and each optical fiber typically has the same cross-sectional structure along its length. The transparent material in one part of the cross-section (usually in the middle) has a higher refractive index than other parts and forms the core in which light is transmitted by total internal reflection. We refer to such fibers as standard fibers.

Due to their superior waveguiding properties, single-mode fibers are preferred for many applications. However, even so-called single-mode fibers generally do not provide sufficient control over the polarization of transmitted light. It is called a single-mode fiber because it only supports a transverse spatial mode at the frequency of interest, but this spatial mode exists in two polarization states; that is, there are two degenerate modes polarized in orthogonal directions. In a real fiber, defects will break the degeneracy of these two modes and modal birefringence will occur; that is, the mode propagation constant β will be slightly different for each orthogonal mode. Since the modal birefringence is generated by random defects, the propagation constant will vary randomly along the fiber. Typically, light introduced into a fiber will travel in both modes and be coupled from one to the other through small bends or twists in the fiber. Linearly polarized light is scrambled to any polarization state as it travels along the fiber.

，β_X和β_Y是正交模的传播常数。这种变化是模的正交分量间的相位差的结果，相差由它们的传播常数间的差别产生。差拍长度越短，光纤对偏振不规则性效应越具有弹性。常规的偏振保留(保偏)光纤典型具有毫米级的差拍长度，双折射的强度也可以参数

表示，其中 (其中λ是波长)，n_X和n_Y是正交模所观测到的折射率。In order to maintain the polarization of the modes in standard fibers, birefringence can be deliberately introduced into the fiber (so that the effective indices (refractive indices) of the two modes are different) so that small defect effects are insignificant. If light is linearly polarized in a direction parallel to one of the optical axes of the fiber, then the light will maintain its polarization. If the light is linearly polarized at other angles as it travels along the fiber, the polarization will change from linear to elliptical to linear to elliptical and back to linear again, with a period of so-called beat length L _B , where

, β _X and β _Y are the propagation constants of the orthogonal modes. This variation is a result of the phase difference between the quadrature components of the modes, which arise from the difference between their propagation constants. The shorter the beat length, the more resilient the fiber is to the effects of polarization irregularities. Conventional polarization-preserving (PM) fibers typically have millimeter-scale beat lengths, and the strength of birefringence can also be parameterized

said, among them (where λ is the wavelength), n _X and n _Y are the refractive indices observed in the orthogonal mode.

In recent years, a non-standard type of fiber called a photonic crystal fiber (PCF) has been demonstrated. The fiber is typically made of a single solid and substantially transparent material into which is embedded a periodic array of air holes parallel to the fiber axis and extending the full length of the fiber. Defects in the form of single air holes lacking in a regular arrangement form regions of increased refractive index in which light is transmitted in a manner similar to total internal reflection in standard optical fibers. Another mechanism for guiding light is based on photonic bandgap effects rather than total internal reflection. Photonic bandgap guidance can be obtained by proper design of the air hole arrangement. Light with a specific propagation constant can be trapped and transmitted in the core.

Photonic crystal fibers are made by packing glass filaments (some of which are capillary on a macroscopic scale) into a desired shape, then melting them together to hold them in place, and drawing them into an optical fiber. PCFs have extraordinary properties such as the ability to transmit light in a single-mode over a very large wavelength range and to transmit light with a sizable mode region that remains single-mode.

Birefringence can arise by several mechanisms. It can arise from the anisotropic nature of the material's susceptibility; ie, anisotropy at the molecular level. It can arise from the arrangement of elements larger than the structure of the material on the atomic scale; this phenomenon is known as shape birefringence. It can also be induced by mechanical stress; this phenomenon is known as stress birefringence or the photoelastic effect. In standard optical fibers, shape birefringence is obtained by changing the shape of the fiber cross-section; for example, by making the core or cladding elliptical. The birefringence in weak waveguide fibers is usually quite weak (B~10 ^-6 ). Stress birefringence is induced in fiber preforms by inserting borosilicate glass rods on opposite sides of the fiber core. Variations in the position and shape of the borosilicate glass rods can induce varying degrees of birefringence. Stress-induced birefringence allows for B ~ 10 ^-4 .

The methods used to generate birefringence in standard fibers, and thus standard polarization-maintaining fibers, are generally not directly applicable to photonic crystal fibers.

The object of the present invention is to provide a birefringent photonic crystal fiber such that the fiber can be used as a polarization preserving fiber. Another object of the present invention is to provide a method for producing the optical fiber.

According to the present invention, there is provided a photonic crystal fiber comprising a bulk material having a longitudinal hole and a guiding core, wherein the fiber has at most two folds of rotational symmetry about a longitudinal axis (which is any longitudinal axis), and as a symmetric As a result, the fiber is birefringent.

Except for the appearance of the core, the arrangement of holes is basically periodic.

Advantageously, the birefringence is that of light having a wavelength of 1.5 micrometers propagating in the fiber having a beat length of less than 1 cm. More advantageously, the birefringence is that of light having a wavelength of 1.5 micrometers propagating in the fiber having a beat length of less than 5 mm. More advantageously, the birefringence is that of light propagating in an optical fiber having 1.5 microns with a beat length of less than 1 mm and preferably less than 0.5 mm; such short beat lengths are generally not available in standard optical fibres. Of course, special fibers cannot guide light at a wavelength of 1.5 microns; in this case, the beat length of the guided wavelength can be easily increased or decreased to be equal to the beat length at 1.5 microns. For example, a beat length of 1 mm at a wavelength of 1.55 microns is equal to a beat length of 0.41 mm at a wavelength of 633 nm, and a beat length of 0.5 mm at a wavelength of 1.55 microns is equal to a beat length of 0.21 nm at a wavelength of 633 nm.

It should be understood that in practical optical fibers there are inevitably less irregularities in the structure, which means that no optical fiber can have any kind of absolute symmetry; however, in conventional photonic crystal fibers, it is clear that practical Fibers do have a considerable amount of rotational symmetry (most commonly six-fold rotational symmetry), and this symmetry is strong enough to form fiber properties similar to those of an ideal fiber with absolute symmetry. Similarly, where reference is made to fibers having up to two folds of rotational symmetry, it should be understood that the fibers not only do not strictly have any higher symmetries, but are not equivalent to fibers having a substantial amount of higher symmetries.

In its broadest aspect, the invention is concerned with the lack of high rotational symmetry in any aspect of optical fiber. More typically, a lack of symmetry occurs in the features of the microstructure inside the fiber, and often in the arrangement of the holes, while the overall cross-sectional shape of the fiber may be circular and thus have circular symmetry; with multiple It is within the scope of the invention to arrange holes with double rotational symmetry, in the absence of more than double rotationally symmetrical fibers and their arrangements. Examples of such arrangements are given below.

The optical fiber preferably has twofold rotational symmetry.

The rotational symmetry is preferably symmetry about an axis through the core.

If the fiber has more than twofold rotational symmetry, then linearly polarized light should have the same propagation constant β when the polarization is parallel to two or more (not necessarily orthogonal) axes. As is often the case in real fibers with circular symmetry, imperfections in the fiber cause power transfer between polarization modes to be parallel to each axis. Therefore, light that is initially linearly polarized excites additional modes and quickly becomes randomly polarized.

The core may include a hole. The hole may be filled with a material other than air. Alternatively, the core may not include holes.

The arrangement of holes can have up to twofold rotational symmetry parallel to the longitudinal axis of the fiber. Alternatively, the arrangement of holes may have more than twofold rotational symmetry about an axis parallel to the longitudinal axis of the fiber. Rotational symmetry may be about an axis through the core.

The lack of higher rotational symmetry results at least in part from changes in one or more of the following in the cross-section of the fiber: microstructure of the core, diameter of the hole, bulk material, material contained in the hole or shape of the hole . The change in shape is due to deformation caused by stress in the fiber as it is drawn. The absence of higher rotational symmetry may result from a change in the cross-section of the fiber as one of or in conjunction with one or more of the following or with other parameters: microstructure of the fiber core , the diameter of the hole, the bulk material, the material contained in the hole, and the shape of the hole.

A birefringent fiber can have shape birefringence and/or stress birefringence. Although the shape birefringence in standard fibers is insufficient to give the required short beat lengths, the potentially large index contrast in photonic crystal fibers can lead to strong shape birefringence. When, during the drawing process, the stress distribution in the fiber distorts some of the air holes around the fiber core along the axis, giving additional birefringence, new effects not possible with standard fibers were discovered.

And according to the present invention, a kind of production method of birefringent photonic crystal fiber is provided, and this method comprises the following steps:

(a) forming a bundle of filaments, at least some of which are capillaries, the bundle of filaments comprising filaments arranged to form a core region of an optical fiber and filaments arranged to form a cladding region of an optical fiber; and

(b) Drawing the stack of filaments into a birefringent fiber having at most two folds of rotational symmetry about the longitudinal axis.

Birefringence was introduced by modifying the method used to fabricate photonic crystal fiber preforms. The modification of the fabrication process may consist of a reduction of the material symmetry in the periodic packing of the filaments comprising the preform, the material symmetry being reduced to a maximum of two-fold symmetric properties. The structure typically changes the shape of the waveguide modes and the pattern of stress distribution in the photonic crystal structure.

One way to introduce birefringence may be to include different capillaries in the preform on pairs of lattice double symmetry lines. These inclusions can be placed near the core in order to change the shape of the guided mode ("shape birefringence") or they can be placed away from the core, but they are made of materials with different properties, thus changing the shape of the fiber core ("stress birefringence") in the form of the stress distribution. Preforms can be constructed to introduce birefringence, as well as stress and shape birefringence, by forming a substantial portion of the fiber preform with different types of capillaries. The fundamental periodic lattice forming the waveguide cladding may be a simple close-packed arrangement of capillaries of normally the same outer diameter, or it may be an arrangement of capillaries generally having different morphological properties and forming different periodic structures. Square lattices can be formed from capillaries and rods with different diameters. For cladding, square and rectangular lattices are available to naturally build birefringent crystal structures for simplified polarization-preserving design of photonic crystal fibers.

The lack of higher rotational symmetry results at least in part from changes in the inner diameter of the capillary across the packing cross-section, the material forming the filament, the material filling the capillary and/or the outer diameter of the filament.

The filaments may be provided at the vertices of a cladding lattice having up to twofold rotational symmetry with respect to the centers of the filaments arranged to form the core. Capillaries of selected inner diameters may be provided at vertices of a cladding lattice having up to twofold rotational symmetry with respect to the center of the filaments arranged to form the core, capillaries of selected diameters at the vertices of the cladding lattice and at other points The diameters of the capillaries are different.

The substantial number of cladding filaments adjacent to the filaments arranged to form the core may vary.

Birefringence can result at least from stresses that develop in the fiber as it is drawn. The stress can be introduced by the inclusion of filaments at points with up to two-fold rotational symmetry of a material different from the material from which at least some of the other filaments in the lattice are made. This stress can be introduced by the inclusion of capillaries with different capillary wall thicknesses at points of up to two-fold rotational symmetry, the thickness of the capillaries being different from the thickness of at least some other capillaries.

This stress can cause deformation of the hole around the core of the drawn fiber and this defect can lead to birefringence.

This stress can lead to stresses in the drawn fiber core, and these stresses can lead to birefringence.

The lack of higher rotational symmetry results at least in part from the pressurization and/or evacuation of at least one capillary during the drawing of the filament stack.

In any of the methods described above, the rotational symmetry of the filament stack is preferably two-fold rotational symmetry.

And according to the present invention, a production method of a photonic crystal fiber is provided, comprising:

(a) providing a plurality of elongated filaments, each filament having a longitudinal axis, a first end and a second end, at least some of the filaments being capillaries, each capillary having a an aperture extending from the first end to the second end of the filament;

(b) forming the filaments into a filament stack, arranging the filaments so that their longitudinal axes are substantially parallel to each other and to the longitudinal axis of the filament stack;

(c) drawing the stack into an optical fiber while maintaining the bore of at least one capillary in communication with a fluid source at a first pressure, while maintaining a pressure around the capillary at a second pressure, which is different from the first pressure, wherein at the A hole under pressure becomes a different size during the drawing than it would have been without the pressure differential.

In the new method, considerable and controllable changes can occur in the fiber structure as the fiber is drawn; for example, there can also be a controllable expansion of air holes during the drawing process. In state-of-the-art photonic crystal fibers, the desired microstructure is formed on a macroscopic scale and then reduced in size by pulling it into the fiber.

Preferably the tube surrounds the stack of filaments for at least part of its length and the interior of the tube is maintained at the second pressure.

It should be understood that the term "expansion of the air pores" refers to the production of air pores whose size (in a cross-section perpendicular to the longitudinal axis of the capillary) is larger than it would have been in the absence of a pressure differential. In fact, the optical fiber produced by drawing has a much smaller total cross-sectional area than the preform (here filament packing) from which the optical fiber is made, so the air holes in the present invention are generally not "expanded" in absolute terms. ".

Variations in the drawing process can be controlled in two main ways: by using differential pressure applied to specific holes, and preferably by sealing the entire preform in a tube, which is preferably thick-walled and contains silica and drawn into and form part of the final fiber. The tube preferably does not undergo deformation significantly different from what it should undergo in the absence of a pressure differential.

The tube preferably limits the expansion of at least one hole at the first internal pressure.

The filament stack preferably has at most two folds of rotational symmetry about any longitudinal axis. The filament stack can be used to draw a birefringent optical fiber.

During drawing it is preferred to:

the tube is sealed to the first end of the evacuated structure and the second end of the tube is located in the evacuated structure;

at least some of the capillaries pass through the evacuated structure and are sealed to their second ends;

The evacuated structure is substantially evacuated to generate a second internal pressure.

The evacuation structure is preferably a metal tube.

By way of example only, embodiments of the invention are described with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a standard fiber embodiment.

Figure 2 is a schematic diagram of a conventional photonic crystal fiber with high index core defects.

Figure 3 is a schematic diagram of a conventional photonic crystal fiber (photonic bandgap fiber) with low index core defects.

Fig. 4 is a schematic diagram of a photonic crystal fiber preform partially drawn into a fiber.

Fig. 5 is a schematic cross-sectional view of a first polarization preserving photonic crystal fiber according to the present invention, wherein the cladding holes form a rectangular lattice.

Fig. 6 is a schematic cross-sectional view of a second polarization-preserving photonic crystal fiber according to the present invention, wherein the pattern of cladding holes near the core has two-fold symmetry.

7 is a schematic cross-sectional view of a third polarization-preserving photonic crystal fiber according to the present invention, wherein the pattern of cladding holes away from the core has two-fold symmetry.

8 is a schematic cross-sectional view of a fourth polarization-preserving photonic crystal fiber according to the present invention, wherein the pattern of dielectric inclusions in the lattice core has two-fold symmetry.

9 is a schematic cross-sectional view of a filament arrangement for forming a photonic crystal fiber with a square lattice.

10 is a schematic cross-sectional view of a portion of a photonic crystal fiber having a square lattice of holes, each hole having one of two different diameters.

Figure 11 shows a photonic crystal fiber with a square lattice.

Figure 12 shows filaments forming part of a filament stack used to form a photonic crystal fiber.

FIG. 13 shows a photonic crystal fiber formed from a stack of filaments such as described in FIG. 12 .

Figure 14 shows a capillary stack suitable for use in another method according to the invention.

FIG. 15 shows the apparatus used with the capillary stack of FIG. 14 .

Fig. 16a shows the cleaved end face of a photonic crystal fiber made from a preform similar to Fig. 14 and the device in Fig. 15 .

Fig. 16b shows a detailed view of the structure near the fiber core of Fig. 16a.

Figure 17a shows a highly birefringent fiber made from the device of Figure 15 .

Figure 17b shows the polarization beat observed at a wavelength of 1550 nm in the fiber of Figure 17a.

Standard optical fibers, such as the example in FIG. 1 , in their simple form basically comprise a cylindrical core 10 and a concentric cylindrical cladding 20 . The core and cladding are typically made of the same material, usually silica, but the core and cladding are doped with other materials to increase the index of refraction of the core 10 and decrease the index of refraction of the cladding 20 . Light of the appropriate wavelength is bound to the core 19 and transmitted there by total internal reflection at the core-cladding boundary 15 .

A typical photonic crystal fiber, shown in Figure 2, comprises a cylinder of transparent bulk material 30, such as silica, having a lattice of cylindrical holes 40 along its length. The holes are arranged at the vertices and centers of a regular hexagon with sixfold rotational symmetry. The holes have a regular periodicity which is broken by defaulting a hole near the center of the fiber. The fiber region 50 surrounding the lattice point lacking holes has the refractive index of the bulk material 30 . The refractive index of the rest of the fiber is due to the refractive index of the bulk material 30 and the air in the hole 40 . The refractive index of air is lower than that of, for example, silicon dioxide, therefore, the "effective refractive index" of a material having pores is lower than the refractive index of the region 50 surrounding the lack of pores. The fiber thus confines light approximately to region 50 in a manner similar to that of a total internal reflection waveguide in a standard fiber. Region 50 is therefore referred to as the "core" of the photonic crystal fiber.

In another form of photonic crystal fiber, the photonic bandgap guide acts to tie the light beam to the "core" of the fiber. In the embodiment of the optical fiber shown in FIG. 3 , there is a matrix of holes 70 in the bulk material 30 . The holes are arranged at the vertices (not in the center compared to FIG. 2 ) of a regular hexagon with six-fold rotational symmetry. The regularity of the matrix is broken again by defects, but in the example it is an additional hole 60 in the center of the hexagon of the lattice, which is located near the center of the fiber. The area surrounding the additional hole 60 is again referred to as the core of the optical fiber. Regardless of the holes 60 (temporarily), the periodicity of the holes in the fiber results in a band gap that may exist in the propagation constant of light transmitted in the fiber. The additional hole 60 effectively creates a region of different periodicity and which can support some propagation constant than is supported in the rest of the fiber. If the propagation constants supported in the region of the hole 60 enter into the bandgap of the propagation constants prohibited in the rest of the fiber, then light with these propagation constants is confined and transmitted in the core. Note that total internal reflection effects are not responsible for the waveguide in this example since hole 60 is a low index defect (which causes air to displace bulk material).

Photonic crystal fibers can be produced by a method, one stage of which is shown in FIG. 4 . In a first stage of the method (not shown), a cylinder of bulk material (eg, silica) is ground to have a hexagonal cross-section and a hole is drilled along its center. The rods are then drawn into filaments using a fiber drawing tower. The filaments are cut into lengths and the short resulting filaments 80 are stacked to form a filament pile, as shown in FIG. 4 . The filament 100 at the center of the arrangement shown is not a capillary, ie it has no holes; the arrangement shown would form an effective index guiding type of fiber. The arrangement of filaments 80 is fused together and then drawn into the final photonic crystal fiber 110 .

The fiber of Figure 5 has a lattice 120 of holes arranged at the vertices of a rectangle, which is not a square. The periodicity of the lattice is broken by the absence of holes in the region 125 near the center of the fiber cross-section. The center-to-center spacing (pitch) of the holes in a direction parallel to the X axis (Λ _x ) is different from the spacing parallel to the Y axis (Λ _y ). The optical fiber shown in Figure 5 can be produced by using filaments ground to have a rectangular cross-section. The lattice of Figure 5 has double rotational symmetry and is therefore birefringent.

6 and 7 show a photonic crystal fiber which is an effective index waveguide fiber, which has a hexagonal shape similar to the fiber of FIG. 2 . The lattice is not birefringent in nature. However, in the lattice of FIGS. 6 and 7 , the diameter of the holes 140 is larger than the diameter of 130 . This armisotropy in the lattice forms a doubly rotationally symmetric pattern of holes about region 135 where one hole disappears from the lattice.

The pattern of large holes 140 in Figure 6 has an effect similar to that of shape birefringence in standard optical fibers. The change in diameter of the hole near the "core" 135 directly contributes to the change in the effective index of refraction observed by the waveguide modes.

The pattern of large holes in Figure 7 creates stress in the core that creates birefringence in the same way that standard optical fibers create birefringence. A novel effect not possible with standard fibers is that during the drawing process, the stress profile (mode) in the fiber can distort some of the air holes around the fiber core 135 along the axis, giving additional birefringence.

Another approach, shown in Figure 8, is to fill some of the holes 150 with material instead of air (so that they have different dielectric constants). Again, the six-fold rotational symmetry of the lattice is reduced to the two-fold rotational symmetry.

The filament stack shown in FIG. 9 is of three types: large diameter filaments 160 which are capillaries, small diameter solid (solid) filaments 170 and large diameter solid filaments 180 . The filaments are arranged so that the large diameter filaments 160 form a square lattice broken by defects at the central lattice point, which are large diameter solid filaments 180 . The interstitial gaps created by the non-mosaic nature of the circular cross-section of the filaments 160 are filled by the small diameter filaments 170 .

Figure 10 shows a photonic crystal fiber with two-fold symmetry. The optical fiber has a lattice structure that can be built up of filaments arranged in a stacking manner similar to that of FIG. 9 . Solid filament 180 causes a defect similar to defect 210 . However, in this case the alternating rows (190, 200) of holes have large and small diameters respectively. Said effect with the lattice of FIG. 9 is obtained by providing alternating rows of filaments 160 with large and small inner diameters (but with a constant outer diameter).

The optical fiber of FIG. 11 can be viewed as approximately having a square lattice such as can be produced from the filament stack of FIG. 9 .

Figure 12 shows an accumulation of filaments 220 which are capillaries. The filaments are arranged in a hexagonal lattice, the periodicity of this structure being broken by solid filaments 240 . It should be noted that the row of filaments about halfway across the photograph are capillaries with thicker walls 250 than the walls 230 of the other capillaries. When an optical fiber is drawn from a stack of filaments, the arrangement results in an optical fiber such as that shown in Figure 13, which has a row of holes 260 that are smaller in diameter than the other holes in the fiber.

Many other patterns of capillaries and filaments with varying parameters can be devised within the scope of the present invention.

Another method of fabricating an optical fiber is shown in FIGS. 14 and 15 . A regular array of capillaries 300 is placed in a thick-walled quartz glass tube 310 (FIG. 14). The quartz glass tube 310 forms part of the optical fiber after drawing and serves as the outer casing to provide mechanical strength. During the drawing process ( FIG. 15 ), the interior of the tube 310 is evacuated by sealing it in a hermetic configuration, whereas, for example, the interior of some or all of the capillaries 300 is kept at a different and higher pressure because they was left open to the air.

The evacuated structures are copper pillars 320 . Initially it is opened at both ends. It is then sealed to tube 310 at one end thereof. The tube terminates in a copper post 320 . Some or all of the capillaries 300 pass right through the copper post 320 and the post is then sealed around the capillary at the upper end just past the post. Copper pillars 320 are evacuated during the drawing process.

During the drawing process, in which the tube 310 and capillary 300 are drawn down from the copper tube, the outer tube 310, despite being evacuated, does not collapse due to having a thick wall. In contrast, the interstitial pores between capillaries 300, which are already small and have relatively thin boundaries defined by capillary walls, collapse rapidly and are absent in the final fiber (which is desired). The evacuated capillary also collapses completely if there is a higher pressure around the capillary. On the other hand, capillaries filled with atmospheric air expand.

By employing the method just described, it is possible to form very regular and thin-walled structures and to fabricate optical fibers with very small waveguide cores. Figure 16 shows the optical fiber as drawn with an outer cladding 330 comprising tubes 310 and an inner cladding 340 comprising capillaries. The inner cladding has a radius of approximately 10 μm and contains a honeycomb structure of expanded pores. These holes surround a waveguide core 350, which is approximately 1 μm in diameter and has been formed from elongated filaments that are not capillaries. It should be appreciated that the fiber in Figure 16 is formed by passing all of the capillaries 300 exactly through the cylinder 320, and that the fiber has substantially multiple rotational symmetry; the fiber is therefore not primarily birefringent.

In contrast, Figure 17a shows the formation of an optical fiber with higher birefringence by packing thick-walled capillaries at specific locations; small air holes 360 are formed at those locations. Another way to produce the fiber could be to have four selected capillaries terminate in the cylinder 320; The fiber of Figure 17a is highly birefringent due to only two-fold symmetry caused by four small holes 360 at either end of the core along the diameter of the inner cladding.

Figure 17b shows polarization beat data for the fiber of Figure 17a. From this data, the beat length of the fiber at a wavelength of 1550 nm can be shown to be 0.92 mm; said beat length is short enough for a fiber to function as a polarization preserving single mode photonic crystal fiber.

Claims (43)

1. photonic crystal fiber comprises and has the block materials that vertical hole and waveguide cores are arranged, this optical fiber result of having maximum double rotation symmetries and lack about the longitudinal axis wherein as symmetry, and this optical fiber is birefringent.

2. photonic crystal fiber as claimed in claim 1, wherein except the appearance of fibre core, the arrangement in hole is periodic basically.

3. as the photonic crystal fiber of claim 1 or 2, wherein birefringence is to make the light with 1.5 microns of wavelength that transmits in optical fiber have the beat length less than 5mm.

4. as the photonic crystal fiber of previous arbitrary claim, wherein this optical fiber has double rotation symmetry.

5. as the photonic crystal fiber of previous arbitrary claim, wherein rotation symmetry be about by fibre core spool.

6. as each photonic crystal fiber of claim 1 to 5, wherein fibre core comprises a hole.

7. photonic crystal fiber as claimed in claim 6, wherein fill with the material that is not air in this hole.

8. as each photonic crystal fiber of claim 1 to 5, wherein fibre core does not comprise the hole.

9. as the photonic crystal fiber of previous arbitrary claim, the arrangement of its mesopore has maximum double rotation symmetries about the axle that is parallel to the optical fiber longitudinal axis.

10. as each photonic crystal fiber of claim 1 to 8, the arrangement of its mesopore has about the axle that is parallel to the optical fiber longitudinal axis and is higher than double rotation symmetry.

11. as the photonic crystal fiber of previous arbitrary claim, wherein higher rotational symmetric lacking to small part, produced by the variation across the fibre core microstructure of cross section of optic fibre.

12. as the photonic crystal fiber of previous arbitrary claim, wherein higher rotational symmetric lacking to small part, produced by the diameter variation across the hole of cross section of optic fibre.

13. as the photonic crystal fiber of previous arbitrary claim, wherein higher rotational symmetric lacking to small part, produced by the variation in block materials across cross section of optic fibre.

14. as the photonic crystal fiber of previous arbitrary claim, wherein higher rotational symmetric lacking to small part, produced by the variation that is included in the material in the hole across cross section of optic fibre.

15. as the photonic crystal fiber of previous arbitrary claim, wherein higher rotational symmetric lacking to small part, produced by the variation across the shape in the hole of cross section of optic fibre.

16. as the photonic crystal fiber of claim 15, wherein change of shape is owing to the distortion that stress caused in the optical fiber when drawing produces.

17. as the photonic crystal fiber of previous arbitrary claim, wherein higher rotational symmetric lacking by the variation across cross section of optic fibre, produce, this variation be a kind of variation in following in conjunction with one or more variations in following or in conjunction with the variation of other parameter: the microstructure of fibre core, the diameter in hole, block materials, be included in the material in the hole, the shape in hole.

18. as the photonic crystal fiber of previous arbitrary claim, wherein birefringence fiber has structural birefringence.

19. as the photonic crystal fiber of previous arbitrary claim, wherein birefringence fiber has stress birefrin.

20. the production method of a double refraction photo crystal optical fiber, this method comprises the following steps:

(a) filament is formed filament and pile, at least some filaments are kapillaries, and this filament heap comprises that arrangement is with filament that forms the fiber core zone and the filament of arranging with formation fibre cladding zone.

(b) the filament heap is pulled into birefringence fiber, this optical fiber has maximum double rotation symmetries about arbitrary longitudinal axis.

21. as the method for claim 20, wherein the longitudinal axis that is aligned to about this accumulation of the accumulation of filament has maximum double rotation symmetries.

22. as the method for claim 20 or 21, wherein higher rotational symmetric lacking to small part, produced by the variation across the capillary inner diameter of piling up xsect.

23. as each method of claim 20 to 22, wherein higher rotational symmetric lacking to small part, produced by the variation across the material that constitutes filament of piling up xsect.

24. as each method of arbitrary claim 20 to 23, wherein higher rotational symmetric lacking to the variation of small part by the material of filling across the kapillary of piling up xsect, produce.

25. as each method of arbitrary claim 20 to 24, wherein higher rotational symmetric lacking to small part, produced by the variation across the filament external diameter of piling up xsect.

26. as each method of arbitrary claim 20 to 25, wherein filament is provided at the place, summit of covering dot matrix, this dot matrix has maximum double rotation symmetries about the center of arranging the filament that forms fibre core.

27. as each method of claim 20 to 25, wherein the kapillary of the internal diameter of Xuan Zeing is provided at the place, summit of covering dot matrix, this dot matrix has maximum double rotation symmetries about the filament center that arrangement forms fibre core, and the selection internal diameter capillaceous that is positioned at covering dot matrix summit place is different from the internal diameter capillaceous at other place.

28. as each method of claim 20 to 27, the quantum of wherein arranging near the covering filament the filament that forms fibre core is different from the quantum of the covering filament of the distant place of arranging the filament that forms fibre core.

29. as each method of claim 20 to 28, wherein birefringence to small part is produced by the stress that forms in optical fiber when the drawing optical fiber.

30. as the method for claim 29, wherein stress is by introducing in the filament inclusions with maximum double rotations symmetries place, this filament is made by a kind of material, and this material is different from the material that forms some other filaments in the dot matrix at least.

31. as the method for claim 29, wherein stress is by introducing in the kapillary inclusions with maximum double rotations symmetries place, this kapillary has a kind of capillary wall thickness, and this thickness is different from some other thickness capillaceous at least.

32. as each method of claim 29 to 31, wherein stress causes being centered around the distortion in the hole around the fiber core of drawing, and this distortion causes birefringence.

33., wherein stress occurs in the fiber core that stress causes drawing, and this stress causes birefringence as each method of claim 29 to 31.

34. as each method of claim 20 to 33, wherein rotational symmetric lacking to small part, produced by one of them pressurization capillaceous in the process that lifts the fiber heap.

35. as each method of claim 20 to 34, wherein rotational symmetric lacking to small part, produced by one of them capillaceous finding time in the process that lifts the fiber heap.

36. as each method of claim 20 to 35, wherein the rotation symmetry of filament heap is double rotation symmetry.

37. the production method of a photonic crystal fiber comprises:

(a) provide the filament of a plurality of elongations, each filament has the longitudinal axis, first end and second end, and at least some filaments are kapillaries, and each kapillary all has a hole that is parallel to the longitudinal axis and extends to second end of filament from first end of filament;

(b) filament is formed heap, arrange filament, their longitudinal axis is parallel to each other substantially and is parallel to the longitudinal axis of heap;

(c) will pile drawing optic fibre, the fluid source contact that keeps at least one hole capillaceous and first pressure simultaneously, keep kapillary pressure on every side at second pressure simultaneously, this second pressure is different from first pressure, wherein becomes in pulling process in the hole of first pressure to be different from the size that it can become when not having pressure reduction.

38. as the method for claim 27, wherein a kind of pipe is piled around filament on a part of length at least, and the inside of pipe is maintained at second pressure.

39. as the method for claim 38, wherein this pipe has limited at least one expansion in the hole of first internal pressure.

40. as each method of claim 37 to 39, wherein this pipe does not experience and significantly is different from the distortion that it should experience when not having pressure reduction.

41. as each method of claim 37 to 40, wherein in pulling process:

Be sealed to first end of the structure of can finding time near first end of pipe, and second end of this pipe is arranged in the structure of can finding time;

At least some kapillaries are by finding time structure and be sealed to its second end;

And this structure of can finding time is evacuated basically, presses to produce in second.

42. as the method for claim 41, wherein this structure of can finding time is a metal tube.

43. as each method of claim 37 to 42, wherein the filament heap has maximum double rotation symmetries about arbitrary longitudinal axis.

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